1. Field of the Invention
The present invention relates to an active-matrix liquid-crystal display and its liquid-crystal driver, particularly to a technique to be effectively applied to a reference voltage generation circuit for generating a gradation display voltage.
2. Description of the Related Art
For example, the specification of U.S. Pat. No. 2,837,027 discloses a conventional liquid-crystal display. FIGS. 11 to 13 show a relation of the connection of an input/output signal between driver ICs of the conventional liquid-crystal display. In general, the connection between driver ICs is performed through a printed wiring board as shown in, for example, FIG. 13.
FIG. 11 shows a state in which a conventional driver IC (liquid-crystal driver) is mounted on a TCP (Tape Carrier Package). An input/output signal is connected between driver ICs by setting an input/output external connection terminal portion 51 common to a plurality of driver ICs to the downside (opposite side to external connection terminal portion 55 for liquid-crystal driving output) of the TCP and as shown in FIG. 13, connecting the terminal portion 51 with a lead terminal for connection of printed wiring boards 71, 72, and 75 by solder.
A driver chip 57 is set to almost the center of the TCP and the external connection terminal portion 55 for liquid-crystal driving output is set to the upside and the input/output external connection terminal portion 51 (common to the plurality of driver ICs) is set to the downside to lead terminals S1 to S7 to the outside. The chip portion is covered with a resin and thereby electrically and physically protected. Moreover, the external connection terminal portion 55 for liquid-crystal driving output is generally directly connected to a liquid-crystal panel through an anisotropic conductive sheet. Because a slit from which a TCP base material is extracted is formed on the input/output external connection terminal portion 51, it is possible to supply a signal common to the plurality of driver ICs by solder-connecting the portion 51 to the printed wiring board.
FIG. 12 is an enlarged view of the connective portion between the chip 57 and the TCP. A pad 67 set on the chip and an inner lead 64 set to the central portion of the TCP are electrically and physically connected each other by thermally contact-bonding them. In this case, the terminals S1 to S7 of the input/output-signal terminal portion 51 are used for signals one each and as a matter of course, pads are used for the signals one each.
FIG. 13 is an illustration showing a mounted conformation of a conventional liquid-crystal module. When assuming a panel of 640 (lateral direction)×400 (longitudinal direction) dots, eight segment drivers vertically arranged have 160 liquid-crystal driving outputs and four common drivers arranged at the left side have 100 liquid-crystal driving outputs.
Moreover, the above specification of U.S. Pat. No. 2,837,027 discloses a method for constituting the liquid-crystal display by only the liquid-crystal panel and TCP without using the above printed wiring board. FIG. 14 shows a state in which driver ICs of the liquid-crystal display are mounted on the TCP. External connection terminal portions for the same input/output signals (S1 to S7) 11 and 12 are arranged at the right and left of the TCP and a slit 13 from which the TCP base material is removed is formed on the external connection terminal portion at one side (left side 11 for this embodiment) and a lead 14 which can be solder-connected is formed on the external connection terminal at the opposite side (right side 12 for this embodiment). Thereby, a configuration is shown in which adjacent ICs are directly connected each other without through the printed wiring board.
FIG. 15 is an enlarged view of the connective portion between a chip 17 and the TCP in the driver IC. The chip 17 is set to a hole portion 20 in FIG. 14. FIG. 15 is greatly different from FIG. 12 in that a pad 27 for the same signals (S1 to S7) is set to the right and left in the chip and the pads 27 for the same signals at the right and left of the chip 17 are connected each other by a wiring material 21 in the chip at a comparatively low impedance. The wiring material 21 is constituted by a conductor such as a second-layer metal on the chip or a gold bump (formed on pad portion of TCP product) on the chip.
A pad 28 for a liquid-crystal driving-output signal 23 is formed on the upper portion of the chip 17. No pad is basically set on the lower portion of the chip 17. However, a dummy pad may be set in order to protect the connection strength between the chip and TCP.
FIG. 16 shows a specific connection procedure between ICs of the driver ICs. The external connection terminal at the slit-13b side of a TCP 40b is set to the upper side and the external connection terminal at the connection lead-14a side of an adjacent IC 17a (40a) is set to the lower side, they are aligned, and leads of the both external connection terminals are overlapped and connected by solder.
FIG. 17 shows a formed liquid-crystal module and a connection between the liquid-crystal and the TCP. A dot configuration (640×400) completely the same as that in FIG. 13 is imaged, in which eight segment drivers using a printed wiring board at upper and lower portions of a panel (four upper drivers and four lower drivers) and four common drivers are used at the left of the panel. Also in this case, a segment driver has 160 liquid-crystal driving outputs and a common driver has 100 liquid-crystal driving outputs.
Devices of eight segment drivers and four common drivers are mutually solder-connected by connection leads 31, 32, and 35 formed at an adjacent overlapped TCP portion. That is, six portions (three upper portions and three lower portions) are mutually solder-connected between the segment drivers and three portions are mutually solder-connected between the common drivers. Moreover, it is possible to connect the common drivers with the segment drivers by the same method.
An example relating to the drain driving circuit of a TFT liquid-crystal display capable of displaying multicolor of 64 gradations in the above driver IC is described in “Low-Power 6-bit Column Driver for AMLCDs”, issued in June, 1994, SID 94 DIJEST pp. 351-354.
The drain driving circuit has one-gradation-voltage generation circuit and generates gradation voltages of 64 gradations in accordance with gradation reference voltages (V0-V8) of nine values input from a not-shown internal power supply circuit.
The drain driving circuit captures 6-bit display data values for red, green, and blue by the number of outputs synchronously with a display-data-latching clock signal and moreover, selects gradation voltages corresponding to the display data values out of gradation voltages of 64 gradations generated by the gradation voltage generation circuit in accordance with an output-timing-control clock signal, and outputs the selected gradation voltages to drain signal lines.
Moreover, to prevent the liquid-crystal layer serving as a pixel from deteriorating, the polarity of an output voltage (voltage to be applied to pixel electrode) of the drain driving circuit and the polarity of a voltage to be applied to a not-shown common electrode are reversed on each AC cycle of a DC to AC signal (not shown).
FIG. 18 is a circuit diagram showing a schematic configuration of the gradation voltage generation circuit of the drain driving circuit of the liquid-crystal display.
As shown in FIG. 18, a gradation voltage generation circuit 606 of the drain driving circuit of the liquid-crystal display first generates gradation voltages of 8×8=64 (gradations) by dividing gradation reference voltages of nine values (V0 to V8) input from the internal power-supply circuit into 8 voltages by the DC resistance division circuit 605.
Then, the circuit 606 selects gradation voltages corresponding to the display data values by a selection circuit 113 constituted by 64×b MOS transistors and outputs the voltages to drain signal lines 1 to b.
FIG. 19 is a circuit diagram showing a schematic configuration for one-gradation reference voltage constituted by a gradation reference voltage Vn and a gradation reference voltage Vn−1 (n=1−8) in the gradation voltage generation circuit 606 shown in FIG. 18, which is constituted by the DC resistance division circuit 605 and a circuit for one-gradation reference voltage of the selection circuit 113.
As shown in FIG. 19, the conventional DC resistance division circuit 605 is constituted by dividing resistors 105 to 112 for dividing the gradation reference voltages Vn and Vn−1 (n=1−8) input from the internal power-supply circuit into eight voltages and has a resistance value R.
Recently, however, there is a trend of decreasing the width (picture frame size) of a portion protruded from the glass substrate of the liquid-crystal panel and securing a larger display area at the same module size. Moreover, because the liquid-crystal panel is still high in cost compared to a CRT, a cost-cutting request is very severe.
Under the above situation, to decrease the width of the TCP protruded from a glass substrate, as shown in FIG. 17, a configuration is used in which the liquid-crystal display is constituted by only the liquid-crystal panel and TCP without using a printed wiring board and a signal line is connected between adjacent TCPs to send or receive an input signal by using only a wiring on the TCP or also locally using a wiring on the glass substrate.
However, in the case of a configuration for sending or receiving an input signal by using only a wiring on the TCP or also locally using a wiring on a glass substrate, the followings become problems: increase of the number of input signals or reference power-supply terminals, increase of cost due to increase of the number of input signals or reference power-supply terminals, and wiring resistance of a reference power supply. Particularly, as the liquid-crystal panel increases in size, a wiring resistance is increased due to extension of wirings in various directions and potentials of a reference power supply and the like may be changed between drivers for driving the liquid-crystal panel due to a voltage drop on a wiring. As a result, a display trouble (block separation) or the like may occur.
It is also considered to increase a wiring in diameter by considering increase of the wiring resistance. For example, however, when increasing the diameter of a lead wiring on the TCP or a wiring on the glass substrate, the size of the TCP increases or it is necessary to increase the driver mounting area on the glass substrate. Therefore, the number of panels to be taken from mother glass may decrease or the cost may increase.
In the case of the single drain driving circuit disclosed in FIG. 18, by dividing gradation reference voltages (V0 to V8) of nine values input from an internal power-supply circuit (not illustrated) into eight voltages by the DC resistance division circuit 605, gradation voltages of 8×8=64 (gradations) are generated and the selection circuit 113 is constituted so as to select any one of gradation voltages corresponding to display data by the DA conversion circuit constituted by 64×b MOS transistors and output the selected voltage.
As the liquid-crystal panel increases in size, a drain driving circuit also tends to be increased in the number of outputs. However, when the number of output loads increases, it is necessary to secure a response speed by decreasing the resistance value of the DC resistance division circuit 605 and supplying a more current. In this case, when the number of source signal lines for outputting the same gradation voltage increases in one drain driving circuit, the voltage fluctuation of a gradation reference voltage generation circuit increases. Particularly, brightness unevenness may occur at an intermediate-gradation display portion in which a change of transmittances of the liquid-crystal layer to an applied voltage is large on a display screen.